The present invention relates generally to methods for improving thermoelectric properties of thermoelectric materials by controlling crystallographic texture and grain size, and particularly to constrained deformation of bismuth telluride based materials by processes such as canned/sandwich rolling, plane strain rolling and plane strain channel die compression at temperatures in excess of 80% of the melting point.
The thermoelectric (TE) effect is the phenomenon of conversion of a temperature difference to an electrical voltage, the Seebeck effect, and an electrical voltage to a temperature difference, the Peltier effect. The Seebeck effect was first observed in the 1820s at junctions between dissimilar metals. The ability of a material to convert temperature differences to electrical voltage, measured by the Seebeck coefficient S=ΔV/ΔT (also called thermopower or thermoelectric power), has been exploited in temperature measuring devices, such as thermocouples. However, in order to use this phenomenon for efficient energy conversion between thermal and electrical energy at any temperature T, the property that needs to be maximized is the thermoelectric figure of merit ZT=S2T(σ/κ), where κ is electrical conductivity, κ thermal conductivity, S is the Seebeck coefficient (or thermopower), and T is the temperature.
An efficient thermoelectric material must, therefore, exhibit a combination of high S (typical of semi-conductors), high σ (typical of metals) and a low κ (typical of insulators). This combination of properties is difficult to achieve.
Thermoelectric materials of interest are generally semiconductors or ceramics with limited ductility. As such, they are difficult to process into the net shapes required for application as energy harvesting devices or thermoelectric coolers (Peltier coolers) for solid-state refrigeration applications. However, these materials can be deformation processed at elevated temperatures to form bulk solids. Prior art deformation processes include hot or cold pressing of powders followed by extrusion into billets for final machining to component geometries. The electronic and thermal properties of these extruded materials are anisotropic due to the crystallographic texture imparted during deformation processing, and they are usually mechanically brittle, depending on chemistry and composition. Typically, prior art deformation processing paths are not tailored to optimize both thermoelectric and mechanical properties.
In a temperature range from −20° C. to about 150° C., which is relevant for most heating and refrigeration applications, bismuth telluride (Be2Te3 or bismuth telluride) has the highest figure of merit among currently available bulk thermoelectric (TE) materials. However, solid state refrigeration systems using the thermoelectric phenomenon have energy efficiencies less than 10% and are therefore used only in niche applications.
There is, therefore, a need for new and improved methods for improving thermoelectric properties of thermoelectric materials.